The present invention relates to a downflow reboiler-condenser. More specifically the present invention relates to a downflow reboiler-condenser having liquid distributing means for distributing and introducing a fluid to be evaporated (a vaporizing fluid) uniformly to evaporation passages of a plate fin type heat exchanger core having condensation passages and the evaporation passages juxtaposed alternately via parting sheets, and particularly to a plate fin type downflow reboiler-condenser suitably used in a double distillation column of an air separation plant.
According to air separation using a double distillation column, liquid oxygen present at the bottom of a low-pressure distillation column (hereinafter referred to as low pressure column) or in a vessel communicating with the low pressure column is subjected to indirect heat exchange with an overhead nitrogen gas of a high pressure distillation column (hereinafter referred to as high pressure column) in a heat exchanger located at a middle part of the double distillation column to effect vaporization of a part of the liquid oxygen to form an ascending agas in the low pressure column and also condensation of the nitrogen gas into a liquid to form a reflux in these two distillation columns. Such heat exchanger is generally referred to as a reboiler-condenser.
As the reboiler-condenser, those using plate fin type heat exchanger cores are generally used. The plate fin type heat exchanger core has a multiplicity of heat exchange passages composed essentially of condensation passages and evaporation passages arranged adjacent to one another via parting sheets, and a fluid to be condensed or condensing fluid (i.e., nitrogen gas) which is introduced in the form of gas and a fluid to be evaporated or evaporating fluid (i.e., liquid oxygen) which is introduced in the form of liquid are subjected to indirect heat exchange with each other to effect condensation of the former fluid into a liquid which is withdrawn to a lower part of the heat exchanger and also to effect vaporization or gasification of a part of the latter fluid into a gas which is withdrawn to a lower part or to a lower part and an upper part of the heat exchanger.
FIG. 1 shows a reboiler-condenser using a submerged plate fin type heat exchanger core (i.e. a submerged reboiler-condenser) utilizing the thermosyphon effect. This reboiler-condenser 1 is used as submerged in an evaporating fluid (liquid oxygen LO) collecting in a reservoir 2a located at the bottom of a low pressure column 2. In the reboiler-condenser 1, the inlet ends and outlet ends (the upper ends and the lower ends) of heat exchange passages (evaporation passages) for the evaporating fluid (liquid oxygen LO) are open, and an overhead nitrogen gas GN in a high pressure column 3 is introduced via an upper header 1a into the condensation passages. The liquid nitrogen formed by the condensation in the condensation passage is withdrawn from a lower header 1b. 
The liquid oxygen in the evaporation passages is subjected to indirect heat exchange with the condensing fluid (nitrogen gas GN) in the adjacent condensation passages to be vaporized partly to form oxygen bubbles which ascend along the evaporation passages. The ascending force of this oxygen gas and the difference in the density of the vapor and that of the liquid in the vapor-liquid mixture bring about the thermosyphon effect and form a circulatory flow in the liquid oxygen LO inside and outside the reboiler-condenser 1. Of the oxygen assuming the form of vapor-liquid mixture withdrawn as an ascending stream, the liquid oxygen which did not vaporize returns to the reservoir 2a, whereas the oxygen gas forms an ascending gas in the low pressure column 2, and a part of the gas is withdrawn as a product through a line 4.
Meanwhile, the nitrogen gas GN introduced into the condensation passages is condensed into liquid nitrogen by the indirect heat exchange with the liquid oxygen and is withdrawn from the bottom of the reboiler-condenser 1. While the thus withdrawn liquid nitrogen is introduced as a reflux to the above two columns, it is occasionally withdrawn partly as a liquid product.
The submerged reboiler-condenser 1 utilizing the thermosyphon effect, as described above, is a counterflow type heat exchanger where the condensing fluid and the evaporating fluid form a downward flow and an upward flow respectively. Since the reboiler-condenser 1, as used, is submerged entirely in liquid oxygen, the liquid head of the liquid oxygen isubcools the liquid oxygen flowing from the bottom of the reboiler-condenser 1 to the evaporation passages.
Accordingly, some distance is necessary for the liquid oxygen until it starts boiling or until the temperature of the liquid oxygen is heated by the indirect heat exchange with the condensing nitrogen gas to reach the saturated temperature. This distance occasionally amounts to 20 to 30% of the height of the heat exchanger. That is, the submerged reboiler-condenser 1 has not enough heat transfer surface area to use the heat transfer surface area over the entire height of the heat exchanger.
Further, the liquid head of the liquid oxygen as an evaporating fluid causes a rise in the boiling point of the liquid oxygen as the evaporating fluid, and the temperature difference xcex94T between oxygen and nitrogen is reduced (temperature pinch) as shown in FIG. 2, to lower the quantity of heat to be exchanged on the designed heat transfer surface area. Therefore, it is now necessary to maintain the temperature difference xcex94T at a fixed level in order to maintain the heat load. As a technique for achieving this, the pressure of the condensing nitrogen gas or the operating pressure of the high pressure column is generally increased in such an amount as to cope with the elevation of the boiling point of the liquid oxygen, leading to an increase in power consumption.
In addition, a large amount of liquid oxygen must be stored to allow the reboiler-condenser 1 to function duly, and it takes a long time to start up the system, or a large amount of liquid oxygen is discharged when the reboiler-condenser 1 is stopped, causing waste of power and personnel cost.
In order to eliminate such inconvenience in the submerged reboiler-condenser utilizing the thermosyphon effect as described above, there is proposed a reboiler-condenser utilizing a concurrent heat exchanger, in which an evaporating fluid is vaporized as it flows down from the top of each evaporation passage in the heat exchanger. This type of reboiler-condenser is generally referred to as a downflow reboiler-condenser.
FIG. 3 shows a downflow reboiler-condenser 5 using a plate fin type heat exchanger. A liquid oxygen LO flowing down from a distillation section 2b of a low pressure column 2 further flows down from the top of the reboiler-condenser 5 together with the liquid oxygen supplied by a pump 6 from a reservoir 2a located at the bottom of the low pressure column and is subjected to indirect heat exchange with a nitrogen gas flowing concurrently in adjacent condensation passages to be vaporized partly. The thus obtained oxygen gas is withdrawn from the bottoms of the evaporation passages into the low pressure column 2, while the liquid oxygen which did not vaporize is withdrawn from the bottoms of evaporation passages to collect in the reservoir 2a located at the bottom of the low pressure column. The thus collected liquid oxygen is returned to the top of the reboiler-condenser 5 for circulation by the pump 6. Since the nitrogen side is of the same configuration as described above, the same and like elements are affixed with the same reference numbers respectively, and detailed description of them will be omitted.
As described above, since the downflow reboiler-condenser 5 forms no liquid head in the liquid oxygen to be evaporated, the heat exchanger comes to have substantially uniform temperature difference xcex94T over the entire height thereof, causing evaporation of the liquid oxygen to take place throughout the heat exchanger. This achieves improvement of heat exchange effectiveness, downsizing and cost reduction in the heat exchanger, as well as, reduction of power consumption, starting time, etc.
Referring to the above downflow reboiler-condenser, those of various structures or constitutions have so far been proposed, for example, in Japanese Patent Publication Nos. Hei 5-31042 and Hei 7-31015 and Japanese Unexamined Patent Publication No. Hei 8-61868. In these reboiler-condensers described in the above official gazettes, there are proposed liquid distributing means for carrying out stepwise liquid distribution as liquid distributing structures for supplying liquid fluids to be evaporated uniformly to evaporation passages.
For example, in the reboiler-condenser disclosed in Japanese Patent Publication No. Hei 5-31042, the liquid distributing means for carrying out stepwise liquid distribution is composed of a pre-distribution section and a fine distribution section; the former is formed of orifices, and the latter utilizes distributing actions of hardway finning (serrated finning). Meanwhile, in Japanese Patent Publication No. Hei 7-31015, the liquid distributing means is composed of a pre-distribution section and a fine distribution section; the former employs pipe orifices and the latter utilizes distributing actions of hardway finning (serrated finning). Further, in Japanese Unexamined Patent Publication No. Hei 8-61868, the area fraction of the perforated finning used as the hardway finning is changed stepwise. each of these liquid distributing means disclosed in these official gazettes is integrated into a heat exchanger core by brazing to constitute a reboiler-condenser.
The liquid distributing means housed in an upper part of a plate fin type heat exchanger in the conventional downflow reboiler-condenser, as described above, involves a problem that fabrication of the heat exchanger costs are high, since it is composed of a pre-distribution section and a find distribution section, and that it has an intricate structure where an evaporating fluid assuming the liquid form to be withdrawn from the fine distribution section is allowed to flow down evaporation passages formed adjacent to each condensation passage via guide plates, such as side bars located at the tops of condensation passages.
It is an objective of the present invention to provide a downflow reboiler-condenser having a liquid distributor at the tops of evaporation passages in a heat exchanger core enabling distribution and introduction of an evaporating fluid uniformly and securely into the evaporation passages and also achieving simplification of the structure and reduction in the fabrication cost.
The downflow reboiler-condenser contains a plate fin type heat exchanger core in which a plurality of condensation passages and a plurality of evaporation passages are formed alternately and successively in spaces defined by a plurality of parallel and vertical parting sheets, respectively, and the reboiler-condenser carries out indirect heat exchange via the parting sheets between a gaseous fluid to be condensed (condensing fluid) introduced from an upper lateral side of the condensation passages and a fluid to be evaporated (evaporating fluid) flowing down onto each evaporation passage to effect condensation of the condensing fluid into a liquid and also vaporization of the evaporating fluid into a gas.
In the downflow reboiler-condenser according to one aspect of the present invention, the evaporation passages are each formed to have an upper end opening and a lower end opening; a reservoir communicating with the upper end opening of each evaporation passage is located above the heat exchanger core; and a liquid distributor is located above each evaporation passage to distribute the evaporating fluid collected in the reservoir into the evaporation passages. Otherwise, the reservoir is defined by the upper end openings and the lower end openings of the evaporation passages being provided, with one header having a pipe for introducing the evaporating fluid and another header having a pipe for withdrawing the evaporating fluid, respectively.
According to a second aspect of the present invention, the evaporation passages are each formed to have an upper lateral opening and a lower end opening; a liquid receiver is located around the upper lateral openings; and a liquid distributor is located above each evaporation passage, the liquid distributor distributing the evaporating fluid to be introduced from the liquid receiver into the evaporation passages through the upper lateral openings respectively. Otherwise, the upper lateral openings and the lower end openings of the evaporation passages are provided with a header having a passage for introducing the evaporating fluid in place of the liquid receiver and with another header having a pipe for withdrawing the evaporating fluid, respectively.
According to a third aspect of the present invention, the evaporation passages are each formed to have an upper lateral opening and a lower lateral opening; a header having a pipe for introducing the evaporating fluid is located around the upper lateral openings; another header having a pipe for withdrawing the evaporating fluid is located around the lower lateral openings; and a liquid distributor is located above the evaporation passages to distribute the evaporating fluid to be introduced into the evaporation passages through the upper lateral openings.
Further, the downflow reboiler-condenser is characterized in that each liquid distributor is of a hardway finning, or that the liquid distributor is composed essentially of a hardway finning serving as an upper liquid distributing section and an easyway finning serving as a lower liquid leading section, or that each liquid distributor is composed essentially of an easyway finning serving as an upper liquid inlet section, a hardway finning serving as an intermediate liquid distributing section and an easyway finning serving as a lower liquid leading section or that each liquid distributor is composed essentially of a hardway finning serving as an upper liquid inlet section, a hardway finning serving as an intermediate liquid distributing section and an easyway finning serving as a lower liquid leading section.
The downflow reboiler-condenser is also characterized in that the hardway finning is of a serrated finning or that the easyway finning of the liquid leading section has a fin pitch not longer than the length of the serration of the hardway finning in the liquid distributing section.
It should be noted here that one kind of finning preferably constitutes integrally the hardway finning serving as the upper liquid inlet section and that serving as the intermediate liquid distributing section. The serration length of the hardway finning is preferably not longer than the fin pitch of the finning located in each evaporation passage. The easyway finning in each liquid leading section is preferably of a serrated finning. The easyway finning in the liquid leading section may have a fin pitch equal to, or xc2xd, the pitch of the finning located in each evaporation passage. The upper end portions of the condensation passages present adjacent to the liquid distributors, as well as, the condensation passages lower than the condensing fluid withdrawing header in the case where the evaporating fluid withdrawing header is located at a lower lateral side of the heat exchanger core are preferably defined as dummy passages where no fluid flows.
As has been described above, according to the downflow reboiler-condenser of the present invention, uniform liquid distribution can be achieved securely using a simple structure, thus achieving reduction in the fabrication cost and improvement of heat exchange effectiveness.